The present invention relates to a method of drilling a subterranean borehole, particularly, but not exclusively, for the purpose of extracting hydrocarbons from a subterranean reservoir.
The drilling of a borehole is typically carried out using a steel pipe known as a drillstring that is coupled with a drill bit on its lowermost end. The entire drillstring may be rotated using an over-ground drilling motor, or the drill bit may be rotated independently of the drill string using a fluid powered motor or motors mounted on the drillstring just above the drill bit. As drilling progresses, a flow of drilling fluid is used to carry the debris created by the contact between the drill bit and the formation being drilled during the drilling process out of the wellbore. The drilling fluid is pumped through an inlet line down the drillstring and through the drill bit, and returns to the surface via an annular space between the outer diameter of the drillstring and the borehole (generally referred to as the annulus).
Drilling fluid is a broad drilling term that may cover various different types of drilling fluids. The term ‘drilling fluid’ may be used to describe any fluid or fluid mixture used during drilling and may cover such things as air, nitrogen, misted fluids in air or nitrogen, foamed fluids with air or nitrogen, aerated or nitrified fluids to heavily weighted mixtures of oil or water with solid particles.
The drilling fluid flow through the drillstring may be used to cool the drill bit. In conventional overbalanced drilling, the density of the drilling fluid is selected so that it produces a pressure at the bottom of the borehole (“the bottom hole pressure” or “BHP”), which is high enough to counter-balance the pressure of fluids in the formation surrounding the borehole (often referred to as the “formation pore pressure”). By counter-balancing the pore pressure, the BHP acts to prevent the inflow of fluids from the formations surrounding the borehole into the borehole that is being drilled. However, if the BHP falls below the formation pore pressure, formation fluids, such as gas, oil and/or water may enter the borehole and produce, what is referred to in the drilling industry as a kick. By contract, if the BHP is very high, the BHP may be higher than the fracture strength of the formation surrounding the borehole, and this high BHP may then result in fracturing of the formation surrounding the borehole, which may in turn lead to loss of fluid from the borehole into the formation. Consequently, when the formation is fractured in this way, the drilling fluid may enter the formation and be lost from the drilling process. This loss of drilling fluid from the drilling process may cause a reduction in BHP and as a consequence cause a kick as the BHP falls below the formation pore pressure.
In order to overcome the problems of kicks and/or fracturing of formations during drilling, a process known as managed pressure drilling has been developed. In managed pressure drilling various techniques may be used to control, the BHP during the drilling process. These techniques may include flowing a gas into the borehole in order to reduce the BHP that is created by fluids, mainly drilling fluids in the borehole.